The present invention relates to an actuator for a seat. More particularly this invention concerns an electrically powered adjuster for a motor-vehicle seat.
At least the driver's seat of a motor vehicle is usually displaceable horizontally relative to a support on the vehicle between a rear end position and a forward end position relative to the direction of travel to compensate for drivers of different height and to allow each drive to select that seat position most comfortable for him or her. It is also known to provide a tilting mechanism which automatically tilts the seat upwardly and forwardly as it is displaced from the rear end position toward the front end position, as typically the driver who likes his or her seat close to the wheel is relatively short and also likes the seat to be raised somewhat.
In the above-cited commonly owned patent application an arrangement is disclosed which automatically effects this horizontal displacement and tilting displacement synchronously. In this arrangement a reversible direct-current motor is operated by a reversing switch so that the user can merely push the switch in one direction to advance the seat in the corresponding direction and can push the switch in the opposite direction to similarly displace the seat in the opposite direction. The switch is normally not of the maintain type so that when released the motor is automatically deenergized and stops.
Nonetheless a problem with such an arrangement is that the switch is normally actuated for at least a limited period of time after the seat has reached an end position, as the user does not immediately ascertain that the seat has stopped moving so continues to hold the switch over for a short time. In order to prevent against overloading of the motor and the entire structure the electric motor is therefore normally provided with a thermal overload switch that automatically cuts the motor out when it heats up as a result of straining to displace the seat which can no longer move against an abutment defining an end position. It is also necessary in such an arrangement to overdimension the mechanism that displaces the seat between the rear and tilted-down end position and the forward and tilted-up position in order to insure that prior to automatic switching off of the motor by the thermal overload arrangement the mechanism is not damaged.
It has been suggested to overcome this problem by the provision of limit switches which automatically shut off or reverse the motor when it reaches an end position. Such an arrangement has proven itself so failure-prone in practice that it has never been used widely.